SIVstm was isolated from a rhesus macaque which had developed an AIDS-like disease upon inoculation with stored lymph node tissue of an Asian stump-tailed macaque; the latter monkey had died in 1977 during an epidemic of acquired immunodeficiency and lymphoma at the California Regional Primate Research Center. Nucleotide sequence analysis of PCR-amplified fragments of the LTR and gag regions indicated that SIVstm was a novel member of the SIV/HIV-2 group. Nucleotide sequences of other regions of SIVstm were also determined. Minimum length evolutionary trees were constructed based upon nucleotide variations in the gag and env regions. A similar branching order of divergence was seen in both cases. Previously known SIVs from sooty mangabey and macaques were only 11% - 13% different from one another and 24% - 30% different from HIV-2s. On the other hand, SIVsm was quite divergent from the other SIVs (16% - 17% different) and only 21% - 25% different from the HIV-2s. The data indicated that SIVstm is the oldest SIV of the SIV/HIV-2 group, reported to date and the most closely related SIV to the HIV-2s.